Using any release of Optifine for 1.5.1, MagicLauncher, with or without Forge ModLoader, default texture pack or custom, it doesn't matter. Minecraft will run at 80-150 FPS for about one second and then it locks up and drops immediately to 0 FPS, 0 chunk updates, sometimes it unlocks for a second or two, but mostly it's just one loooong lag spike. CPU drops dramatically when the lag occurs - it seems to just stop processing any data. Minecraft runs fine without Optifine.

I can confirm that Optifine + the latest Forge is completely borked. Missing textures in many mods and occasional OpenAL / GL crashes. And unfortunately many mods are now requiring the latest forge builds because there were some major changes.

I'm running Optifine on a 2007 iMac with an ATI Radeon HD 2600. I have no issues and I've better FPS than with the vanilla client.

However, I just played Minecraft on a friend's iMac. It's the latest model, 8 GB RAM 2GB Nvidia GeForce GTX 680MX, Quad core i7, Fusion Drive (Hybrid SSD/HDD) 2560x1440 27 inch screen. That thing runs Starcraft 2 on Ultra at 80 FPS in native resolution. But it has the same issue as previously described, 1-2 seconds of screen updates then 10-20 seconds of freezing. No setting can make it work, the only option is running the vanilla client.

I'm running Optifine on a 2007 iMac with an ATI Radeon HD 2600. I have no issues and I've better FPS than with the vanilla client.

However, I just played Minecraft on a friend's iMac. It's the latest model, 8 GB RAM 2GB Nvidia GeForce GTX 680MX, Quad core i7, Fusion Drive (Hybrid SSD/HDD) 2560x1440 27 inch screen. That thing runs Starcraft 2 on Ultra at 80 FPS in native resolution. But it has the same issue as previously described, 1-2 seconds of screen updates then 10-20 seconds of freezing. No setting can make it work, the only option is running the vanilla client.

Happy to report that my issues have been resolved with the latest version - OptiFine HD B3 Ultra seems to be running as normal - using Forge build #608 as recommended. Can now play on Fancy and Normal view distance

Happy to report that my issues have been resolved with the latest version - OptiFine HD B3 Ultra seems to be running as normal - using Forge build #608 as recommended. Can now play on Fancy and Normal view distance